In the early 1200s, Genghis Khan united the warring Mongolian tribes into a mobile, efficient military state. Lashing outward in all directions from their home on Central Asia's steppe, the Mongolian armies conquered a large swath of Central Asia in just a few decades. The empire continued to expand under Genghis Khan's descendants and, at its height, was one of the largest in human history, extending from Asia's Pacific coast to Central Europe.

The Great Khan is remembered as a politically savvy leader and a brilliant military tactician, but the rise of his empire, new research suggests, might have also had something to do with a stretch of unusually nice weather.

In 2010, American researchers Neil Pederson and Amy Hessl were in Mongolia's Khangai Mountains, studying the impact of climate change on the country's wildfires. As they drove past an old flow of now-solid lava left by a volcanic eruption thousands of years ago, they saw stands of stunted pine trees growing out of cracks in the lava.

Now, as any budding naturalist can tell you, the annual growth rings of many trees reflect the conditions they grew in. A long, wet growing season results in a wide ring, and a drought-stricken year means a thin ring. After you figure out the age of a tree, these growth patterns can provide a year-by-year record of what the local climate was like. Luckily for Pederson and Hessl, these patterns were written very clearly into the trunks of their Siberian pines, which were well-preserved by the cold, dry conditions of the steppe. The pair had potentially found a wooden record of climate conditions going back thousands of years.

Pederson and Hessl took samples from 17 of the trees and found that they were indeed very old. The innermost rings of some them dated all the way back to the 7th century. Since this discovery, they've gone back and sampled more than a hundred trees in the mountains and the Orkhon Valley region, where Genghis Khan established the seat of his growing empire.

Combining their tree-growth patterns with temperature reconstructions, Pederson, Hessl, and their team pieced together a picture of what the climate was like during the centuries that the Mongols conquered and ruled.

Just before Genghis Khan rose to power, Mongolia's climate was harsh, both physically and politically. The Mongolian tribes warred against each other, and the steppe was cold and stricken by drought. Amid the conflict, the researchers say, the worsening dry conditions of the land could have been an important factor in the collapse of the old order, and paved the way for centralized leadership under Genghis Khan. "What might have been a relatively minor crisis instead developed into decades of warfare and eventually produced a major transformation of Mongol politics," they write.

Then, in the early 13th century, as Genghis Khan unified the tribes, the droughts gave way to a period when the steppes were wetter and warmer than they'd ever been. "This period, characterized by 15 consecutive years of above average moisture in central Mongolia and coinciding with the rise of Genghis Khan, is unprecedented over the last 1,112 years," the researchers say. In addition to being wet, Mongolia at the time was warm, but not exceptionally hot.

In these conditions the Mongolian grasslands would have flourished, providing fuel for the Mongolian war machine. Each of Genghis Khan's mounted warriors used several horses, and the conquering armies brought herds of livestock with them for food and other resources. The dramatic shift in temperature and precipitation came at the perfect time to provide resources for rapid military mobilization and the Mongols' early expansion.

After the empire's initial spasms of growth, the tree ring and temperature data show a return to a cold, dry climate. By then, though, the Mongols had defeated several other Central Asian powers and could exploit the conquered regions instead of relying on the grass of the steppes and their local resources.

The climate shift certainly isn't the only driver of the empire's quick rise; it might have also just been coincidental, the researchers say. To flesh out the picture that the tree rings provide, the team is working on several other studies that could corroborate their ideas. Ecologist Hanqin Tian is developing models to connect the dots between the tree-ring records of weather and grass production. Biologist Avery Cook Shinneman will analyze the layers of fungal spores from animal dung that are trapped in sediment in Mongolian lakes, which could indicate the abundance of the Mongols' livestock. Meanwhile, historian Nicola Di Cosmo will comb through records from Asia and Europe looking for historical references to the climate and the strength of the Mongolian armies.

While the tree rings provide clues about the past climate and its possible influence on the rise of an empire, they also hint that another major shake-up is yet to come in Central Asia. As they did hundreds of years ago, conditions in the region have turned from wet to arid, with long, cold winters and drought-stricken summers comparable to those experienced just before Genghis Khan seized power. During the 2000s, livestock booms went bust; millions of animals died, and hundreds of thousands of displaced herders flocked to the city of Ulaanbaatar.

Those earlier droughts happened in a much cooler climate, though. Central Asia is currently warming more than the global average, and the combination of rising temperatures and droughts, the researchers warn, could mean another era of climate-spurred social and political upheaval.